This invention relates to the manufacture of three-dimensionally molded particleboard parts.
The raw materials used in the manufacture of molded particleboard parts consist of mixtures of lignocellulosic materials combined with heat-curable binders whose rate of cure is accelerated by the application of heat. The mixtures may take the form of loose collections of particles, fiber bundles, flakes, slivers, or shavings. The mixtures may also be compacted to some extent to create molded preforms. The various forms of the raw material for molded particleboard parts will be collectively referred to as the xe2x80x9cfurnishxe2x80x9d throughout this disclosure.
Many different lignocellulosic materials, particle types, and particle sizes are used in the furnish. For example, wood-based furnish can take the form of wood chips, wood-fiber bundles, wood flakes, wood shavings, wood slivers, wood flour, or a variety of wood residues. The most common binders used in the furnish are urea-formaldehyde and phenol-formaldehyde, while isocyanate resins are used only occasionally. These binders are all synthetic heat-curable binders in which curing is accelerated by the application of heat. In most common furnishes, the binder constitutes approximately 2-10% of the furnish, although higher binder percentages are occasionally used.
The furnish is compressed into final shape between matched male and female metal dies that are pressed together and simultaneously heated in a conventional heated press. The female die is known as the cavity and the matched male die is known as the punch. In operation, the shape of the facing surfaces of the punch and the cavity impart three-dimensional features to the finished particleboard parts. Molded particleboard parts are to be distinguished from standard flat-panel composition boards that could be characterized as being only two-dimensional. The present invention does not pertain to two-dimensional flat-panel composition boards, such as these, since these products are not normally molded using cooperating metal dies.
A simple particleboard molding process is used to produce shallow draws on embossed products, such as door skins, cabinet fronts, and embossed wall panels. More sophisticated methods have been developed to mold parts having deep draws, such as cores for upholstered furniture, tabletops with profiled edges, irregular boxes, curved drawer fronts, beverage cases, and toilet seats. Many of the initial steps in producing molded particleboard parts are similar to the initial steps for producing standard flat composition boards. For example, furnishes are prepared in very similar ways. Important differences occur primarily at the forming station, where the furnish is consolidated into its finished form.
All of the current processes for making molded particleboard products require very long furnish dwell times within the heated press, due to the low heat transfer rates from the heated molds to the furnish being compressed. Furnish dwell times vary between approximately one minute to more than 10 minutes, depending upon the part thickness. Since profitability of a particleboard molding operation is closely related to product throughput, these long furnish dwell times within the heated press limit production rates and continue to be a major economic concern to the industry.
At first glance, it may seem possible to remedy this situation and decrease furnish dwell times within the heated press by further heating the fanish with radiowave energy as the furnish is being consolidated in the press. This process is known as in-press radiowave heating. The term, radiowave, by standard definition, is an electromagnetic wave having a frequency between 10 kilohertz up to about 30 gigahertz. Using supplementary radiowave heating, decreased furnish dwell times within the heated press would be expected because radiowaves are known to propagate throughout the volume of dielectric materials, thereby producing rapid volumetric heating in radiowave-absorptive dielectrics, such as wood/binder compositions.
In practice, for most molded particleboard sizes and shapes, radiowaves applied within the forming mold during the hot-pressing operation do not establish uniform electric field distributions throughout the volume of the furnish contained within the mold. This is because only a relatively small number of microwave modes can be established within the restrictive confines of the mold interior. In addition, because the thickness of the molding space between the punch and the cavity bottom is usually much less than a wavelength at commonly used microwave frequencies, the microwave electric fields are polarized primarily in only a single direction roughly normal to the base of the part.
With few microwave modes and only a single field polarization, strong electric field concentrations form at various standing-wave maxima, and microwave fields are attenuated within intricate mold features. High microwave field levels will also be produced near the point where microwaves are fed into the mold interior. Since the parts are held in a stationary position within the mold cavity, these field variations lead to strong non-uniform heating of the compacted furnish and non-uniform curing. In addition to producing non-uniform heating and curing of the furnish, application of microwaves to the mold interior during hot-pressing would require complex new mold designs and replacement of existing molds, making retrofitting of microwave-heated mold cavities to existing molding equipment impractical and uneconomical.
Radiowave frequencies below the microwave range have also been considered for in-press radiowave heating. For the most common industrial frequency bands in this range, wavelengths are much greater than mold dimensions. Radiowave system construction in this frequency range differs considerably from construction in the microwave range. Compared to microwave heating, these relatively low frequencies produce much less heating power within the furnish for a given electric field level, since the power density scales primarily with frequency. To match microwave power densities, much higher electric fields are required at lower frequencies, which can lead to more frequent arcing.
Non-uniform in-press radiowave heating can also occur in this low frequency range just as it does for in-press radiowave heating in the microwave range. For low frequency radiowave in-press heating, non-uniform heating can occur because of fringing fields, field concentrations at sharp mold features, and variations in field levels due to variations in the thickness of the parts. Because the use of low frequency radiowaves for in-press heating also requires replacement of existing molds, the same problems with retrofitting existing particleboard molding operations exist, as were described for in-press microwave heating schemes.
As a consequence of the problems that occur when radiowaves are applied within the mold cavity as the mold is hot-pressed, another solution to the problem of long hot-press dwell times and long production cycles for molded particleboard parts is needed.
The essence of the invention is a heretofore unsuggested use of radiowave energy to speed production of molded particleboard parts by preheating the furnish prior to the application of heat and pressure within the forming mold during the hot-pressing operation. Because radiowaves propagate readily throughout the volume of the furnish, rapid heating is obtainable even for a loose, uncompacted particulate furnish that has very poor thermal conductivity. By utilizing a radiowave preheating scheme, rather than applying radiowaves simultaneously with hot pressing, relatively simple yet highly effective radiowave applicators may be devised. In many cases, standard applicators disclosed in the prior art may be used.
An important precept of the invention is that the temperature of the preheated furnish is kept low enough that the dry-out point of the binder is not reached, and it is still possible to obtain a good bond between fibers in the subsequent hot-pressing operation. The energy applied to the preheated furnish reduces the energy required from the heated press in completing the cure of the molded particleboard part, resulting in reduced furnish dwell times within the heated press.
There are at least two general radiowave preheating scenarios. The first scenario will be referred to as xe2x80x9cexternal preheatingxe2x80x9d and the second scenario will be referred to as xe2x80x9cinsitu preheating.xe2x80x9d In the external preheating scenario, radiowave preheating is performed as a completely independent operation, before the furnish is added to the forming mold and before the furnish is consolidated and cured within the heated press. The furnish is unconstrained in this case, which allows the furnish to be easily translated over distances greater than a wavelength, while radiowaves are applied, assuring uniform exposure of the furnish to radiowave fields and uniform radiowave heating of the furnish. After preheating the furnish with radiowave energy, the furnish is quickly transferred to the cavity of the forming mold. The punch for the forming mold is then inserted into the cavity and the preheated furnish is pressed between the mold dies. The whole operation is carried out as quickly as possible, to avoid precure of the furnish and to assure a good bond between fibers during hot-pressing. In the heated press, additional heat energy is applied to the furnish to complete the cure and solidify the part in its final form.
Since radiowaves are applied before the furnish is added to the hot-press forming mold, the forming mold does not need to include provisions for coupling microwaves into the furnish. In fact, no modification of existing all-steel molds is required. Molded particleboard parts of any size or complexity may be formed, limited only by the size and forming properties of well known conventional steel molds. Radiowave preheating may be readily added as a separate and independent operation, making it easily retrofitted to existing molded particleboard production plants. In addition, because radiowave preheating by the external preheating scenario is a separate operation from the final hot pressing step, various molds may be interchanged in a given press without changing the radiowave preheating equipment.
The second radiowave preheating scenario, insitu radiowave preheating, is similar to the external radiowave preheating scenario, except that the furnish is preheated directly in the cavity of the forming mold prior to compressing the forming mold in the hot press. There are several specific procedures that may be used for insitu radiowave preheating .
For example, one procedure specifies that the punch is partially inserted into the cavity in a standby or ready position while radiowave preheating is applied to the cavity within the heated press. This procedure minimizes the time between preheating and hot-pressing. In this case, some alteration of the forming mold is required. Provisions for feeding radiowave power into the cavity and suppressing radiowave emissions must be added to the to of the cavity portion of the forming mold. To accommodate the extra length added to the cavity by the microwave feed and the radiowave suppression structure, the punch length must be increased. While modifications to the mold cavity and punch are required, these modifications are considerably simpler than the modifications that would be required for in-press radiowave heating schemes.
To avoid the need for modifications of the hot-press forming mold for insitu preheating, an alternative procedure may be used. In this alternative insitu procedure, unheated furnish is first added to the forming mold cavity. Radiowaves are then applied to the unheated furnish within the mold cavity prior to insertion of the punch, while the opening into the mold cavity is unobstructed. Radiowave preheating can then be performed outside of the heated press since the cavity can be separated from the punch. For example, the cavity can be mounted on a sliding platen, moved out of the press for radiowave preheating, and back into the press for hot-pressing and final cure. Utilizing the sliding platen, the entire cavity may be inserted into a multimode radiowave cavity or other radiowave applicator positioned beside the hot press.
Within the multimode radiowave cavity, radiowaves propagate readily to the furnish through the opening in the mold cavity, as long as the dimensions of the opening are on the order of a wavelength or more. No modifications of existing all-steel molds are required for this insitu preheating procedure so that it can be readily adapted to existing particleboard molding operations, as was the case for external preheating scenarios.
In addition, by using a sufficiently large microwave cavity, the furnish contained in several forming cavities may be preheated simultaneously, which would be useful in some operations that press several parts at the same time in the heated press. Time between preheating and hot pressing is minimized in both of the above insitu procedures through the elimination of the furnish transfer step which moves the preheated furnish from the preheater to the cavity of the forming mold.
In both the external preheating scenario and the insitu preheating scenario, as much as ⅔ of the total energy required for curing can be applied during the preheating stage of the invention, as will be described in more detail later in this disclosure. A much lower energy requirement is then placed upon the heated press, resulting in substantially reduced furnish dwell times within the heated press. By performing the radiowave preheating operation in parallel with the hot-pressing operation, with its reduced furnish dwell time, production rates are greatly increased using the preheating concept disclosed herein.
Because of the advantages disclosed in the above discourse, it is apparent that the invention provides a practical and economical approach to reducing furnish dwell times within the heated press and increasing production rates in new and existing molded particleboard plants. Implementation of the teachings of this disclosure will result in a practical and economical means of increasing molded particleboard production rates, reducing manufacturing costs, and improving profitability.
These economic benefits will encourage the manufacture of new molded particleboard parts and improve production efficiencies of existing parts. In addition, the invention will make it economically feasible in many cases to replace plastic parts with parts manufactured from wood particles. Since wood is a renewable resource, unlike the petroleum from which plastics are manufactured, molded particleboard materials should be derivable from more stable raw material sources, and should provide an environmentally attractive alternative to plastic parts.
While radiowave preheating of furnish to speed production of molded particleboard parts has not been previously suggested, radiowave preheating of standard flat-panel composition boards has been suggested and, in fact, implemented in various ways in the particleboard and fiberboard industries. One of the earliest suggestions to use microwaves to accelerate the curing of resinous binders in flat composition boards was disclosed by Pike and Barnes in U.S. Pat. No. 4,018,642.
In this patent, Pike and Barnes mention the use of microwaves to xe2x80x9c. . . heat the resin and accelerate its curing, pressure being applied during the application of microwave energy or shortly thereafter,xe2x80x9d While this statement alludes to the use of a microwave preheating process to accelerate resin curing, the disclosure describes in a definitive way only preheating of flat panel boards having two dimensional characterization. There is no suggestion to use microwave preheating to accelerate the curing of three-dimensionally molded particleboard parts.
Because of their more complex structure, these three-dimensional parts have unique problems associated with radiowave heating that are not encountered in radiowave heating of boards having a two-dimensional characterization. These unique problems were never recognized nor pointed out in the prior art of Pike and Barnes, nor in any other prior art reference. For example, none of the prior art recognizes the problem of inaccessibility of microwaves in the intricate recesses of many three-dimensional molds, which contributes to non-uniform microwave heating of the furnish. Neither does the prior art recognize the problem of large standing waves for in-press microwave heating schemes, also contributing to non-uniform heating and curing of to the furnish. These unique problems discourage the use of in-press radiowave heating, but encourage the use of radiowave preheating outside of the hot press where greater control of heating uniformity can be obtained. Yet none of the prior art references recognize or suggest the advantages of radiowave preheating over in-press radiowave heating in the manufacture of molded particleboard parts.
In fact, Pike and Barnes, teach away from the use of microwave preheating in general as a xe2x80x9cless preferred embodimentxe2x80x9d compared to in-press microwave heating in the formation of flat board products. Yet it has been made clear in this section and in the previous section of the present disclosure that microwave preheating is actually a preferred method for accelerating the manufacture of molded particleboard parts because of the unique features of molded particleboard manufacture.
In-press microwave heating, the preferred embodiment of the invention of Pike and Barnes, is impractical in most situations in the manufacture of molded particleboard parts because of the complexity of the molds, and because retrofitting to existing molding operations requires major equipment modifications. In addition, if the present invention were obvious, those skilled in the art of molded particleboard manufacture surely would have implemented the invention by now. Yet there is no description of implementation of radiowave preheating of molded particleboard parts in any prior art reference, including the very comprehensive survey of the particleboard industry given by Maloney (1993). The fact that the invention has not been implemented in the manufacture of molded particleboard parts, despite the great advantages disclosed in this section, indicates that the present invention is not obvious.